An embodiment of the hybrid collector is disclosed in the U.S. Pat. No. 4,334,120 assigned to the assignee of the present invention. In this embodiment, the hybrid collector comprises a main portion, enclosed in a vacuum vessel exhausted to obtain a substantial adiathermancy, including solar cells attached, directly or through an insulating film, to a heat collecting plate having a heat collecting tube.
Each of the above-mentioned solar cells comprises a metallic lower electrode, an a-Si layer and a transparent upper electrode successively formed in strata. Due to such construction, the light which is absorbed by the a-Si layer of the solar cell is converted into thermal and electrical energies. Furthermore, the thermal energy is outputted from the heat collecting plate through the heat collecting tube. Also, the electric energy is outputtted through the electrodes.
Meanwhile, radiation spectra of sunlight is generally distributed over a wavelength range of about 0.3 .mu.m to 2.5 .mu.m and have a peak near the wavelength of 0.5 .mu.m. On the other hand, it is known that the a-Si layer absorbs a considerable part of the light having wavelengths of less than about 0.8 .mu.m while it scarcely absorbs light having wavelengths longer than 0.8 .mu.m.
Accordingly, by means of the hybrid collector such as disclosed in the above-mentioned embodiment, a part of the incident sunlight, being transmitted into the hybrid collector through a transparent wall, usually formed of a glass, of a vessel enclosing the hybrid collector and having wavelengths of less than 0.8 .mu.m, is effectively absorbed by the a-Si layer whereby it is converted into usable electric and thermal energies. However, light having wavelengths of about 0.8 .mu.m 2.5 .mu.m which may be transmitted, as mentioned above, through the a-Si layer is reflected by the lower electrode and radiated outward so that it is hardly utilized.